Flash lamp q-switched laser system

ABSTRACT

A Q-switch for a solid-state laser system is provided by a gasfilled flash lamp within the optical feedback path of the laser system. One embodiment utilizes the opaque region of an ionized flash lamp to interrupt the optical feedback path during the early portion of the laser rod pumping cycle. Feedback is restored by deionizing the flash lamp. A second embodiment utilizes an auxiliary feedback loop to control the rate at which the intensity of the laser radiation within the feedback system increases. An ionized flash lamp having a light saturable characteristic is included in the primary feedback loop. The auxiliary feedback loop is adjusted such that the ionized flash lamp optically saturates at a predetermined time. Saturation of the flash lamp increases feedback, thereby causing the flash lamp to function as a Qswitch element.

United States Patent Ward [ 51 Oct. 3, 1972 [54] FLASH LAMP Q-SWITCHEDLASER SYSTEM 72 Inventor: Robert w. Ward, Dallas, Tex.

[73] Assignee: Texas Instruments Incorporated,

Dallas, Tex.

[22] Filed: Oct. 30, 1970 [21] Appl. No.: 85,577

[52] US. Cl. ..33l/94.5, 350/ 160 R [51] Int. Cl ..H0ls 3/00 [58] Fieldof Search ..331/94.5; 350/160 [56] References Cited UNITED STATESPATENTS 3,321,714 5/1967 Tien ..33l/94.5 3,493,888 2/1970 Jackson..331/94.5 3,134,837 5/1964 Kisliuk et a1 ..331/94.5 3,500,234 3/1970Goedertier ..331/94.5 3,581,230 5/1971 Smith ..33l/94.5

Primary Examiner-Ronald L. Wibert Assistant Examiner-Edward S. BauerAttorney-James 0. Dixon, Andrew M. l-lassell, Harold Levine and Rene E.Grossman [57] ABSTRACT A Q-switch for a solid-state laser system isprovided by a gas-filled flash lamp within the optical feedback path ofthe laser system. One embodiment utilizes the opaque region of anionized flash lamp to interrupt the optical feedback path during theearly portion of the laser rod pumping cycle. Feedback is restored bydeionizing the flash lamp.

A second embodiment utilizes an auxiliary feedback loop to control therate at which the intensity of the laser radiation within the feedbacksystem increases. An ionized flash lamp having a light saturablecharacteristic is included in the primary feedback loop. The auxiliaryfeedback loop is adjusted such that the ionized flash lamp opticallysaturates at a predetermined time. Saturation of the flash lampincreases feedback, thereby causing the flash lamp to function as aQ-switch element.

6 Claims, 8 Drawing Figures PATENTEDHN 3 m2 SHEET 1 BF 3 lA/l/EA/TO/PRobert W. Ward @MZZ 7 W/TA/ESS ATTORNEY P'A'IENTEDMI; I91:

sum 2 er 3 LIGHT INPUT (IN Mw/CM PATENTEDocI 3 I972 SHEET 3 (IF 3 FLASHLAMP Q-SWITCHED LASER SYSTEM BRIEF DESCRIPTION OF THE INVENTION ANDBACKGROUND INFORMATION A wide variety of laser systems are currentlyknown and used. Most ofthe fundamental differences between these systemsinvolve variations in the light sources and the laser materials used.Some of the laser materials currently used are gases, glass andsemiconductors.

Suitable light sources include various types of flash lamps and arraysof light-emitting solid-state diodes.

A laser system is an oscillator operated in the infrared, the visible,or the ultraviolet portions of the frequency spectrum. The fundamentalportions of any laser system is an amplifier utilizing the principle oflight amplification by stimulated emission of radiation and an opticalfeedback loop in which a portion of the output of the amplifier is fedback into the input of the amplifier for further amplification therebycausing the system to oscillate.

A laser rod in which the atoms have been pumped to a high energy stateby the absorption of light from an external source can emit lightradiation by two processes. The first process is known as spontaneousradiation. This is a process in which atoms of the laser rodspontaneously revert from a high energy state to a lower energy statewithout any further outside stimulus with the difference between theenergy of the two states being radiated as light. The second process isknown as stimulated emission of radiation. This is a process in whichlight radiation impinges upon atoms of the laser rod which have beenpumped to the high energy level by absorption of light from an externalsource. This light stimulates these atoms to change from their highenergy state to a lower energy state with the difference between the twoenergy states being emitted as light radiation. The wavelength of thelight resulting from the stimulated emission will be the same as thelight stimulating the emission.

In typical systems the portion of the output resulting from spontaneousradiation is small as compared to the output resulting from thestimulated emission of radiation process. However, it should beemphasized that without spontaneous radiation no oscillation would occurand the system would be inoperative.

In typical laser systems, a high intensity light source is used toilluminate a laser rod. Portions of the light energy from the source isabsorbed by the laser rod causing the atoms of the laser rod to changefrom their lower energy state to a higher energy state. Mirrors arepositioned such that the radiation which is emitted along thelongitudinal axis of the laser rod by spontaneous radiation will bereflected back through the laser rod causing more light to be emitted bythe stimulated emission process. Each time this radiation is reflectedback through the laser rod, the output increases due to the stimulatedemission process. The intensity of the light in the feedback loopcontinues to build up through this process until an equilibrium 'pointis reached between the amount of energy absorbed from the light sourceby the laser rod and the amount being radiated by the laser rod in theform of either spontaneous or stimulated emission. Useful output of thelaser system is obtained by making one of the mirrors in the feedbackloop only partially reflective so that a portion of the light will betransmitted through the mirror as a high intensity light beam with asecond portion being reflected back into the feedback loop to maintainthe emission process.

In the above described systems, the light source in conjunction with thelaser rod constitute the amplifier. The mirrors form a feedback systemcoupling a portion of the amplifier output back to the amplifier input.

Lasers operate in two fundamental modes. These modes are the continuousmode in which the light output is uninterrupted and the pulse mode inwhich the output is a seriesof short pulses. The preferred mode ofoperation will depend on the application for which the laser system isdesigned. If the application indicates that a pulsed laser system shouldbe used and that the output pulse should be a high energy pulse ofrelatively short duration, the current state of technology requires thatthe laser system include some type of Q-switch element.

A Q-switch element is an element placed in the opti- .cal feedback pathof the laser system and having characteristics which permit the opticalfeedback of the laser system to be interrupted during the early portionsof the laser rod pumping cycle. This interruption of the feedback pathassures that the laser rod will store sufficient energy to emit a highpeak power pulse at the time the resonatorQ is switched and thestimulated emission process occurs.

Typical prior art Q-switch elements include the Kerr cell, the Pockelscell, rotary or vibrating reflectors, and saturable absorbers. The Kerrand Pockels cells are electro-optic devices while prior art saturableabsorbers are typically organic'dyes which become more transparent withincreasing levels of light.

The electro-optic devices, such as the Kerr and Pockels cells, aremechanically fragile and require high voltage sources. The mechanicalQ-switch elements such as the rotating or vibrating reflectors havemechanical problems due to the high rate at which the reflector mustrotate or vibrate. Prior art saturable absorbers such as organic dyesinvolve chemical reactions which are not totally reversable. Theseundesirable characteristics have made prior art Q-switch elements lowreliability, high maintenance items.

The laser system accordingto the present invention advantageously solvesthe problems associated with prior art Q-switched lasers by providing aQ-switched laser system in which the Q-switching element is a flashlamp. The Q-switch elements utilize either the light blocking or thesaturable absorber characteristics of an ionized gas-filled flash lampoften referred to as a laser flash tube.

In one embodiment, the Q-switch element is a flash lamp positionedwithin the optical feedback path of the laser system. A second flashlamp is used as a light source to optically pump the laser rod. Twovoltage sources are used, one for the flash lamp which is used as theQ-switch element and one for the light source used to optically pump thelaser rod.

Another embodiment provides a laser system in which a single flash lampis used as the light source to optically pump the laser rod and also asthe Q-switch element.

A still further embodiment includes a second or auxiliary opticalfeedback path which causes the laser radiation in the feedback loop tobuild up at a controlled rate such that a flash lamp in the opticalfeedback path is caused to optically saturate, at a predetermined time,thereby causing the feedback to increase. The rate at which the laserradiation builds up is varied by adjusting the auxiliary opticalfeedback loop. This loop is preferably adjusted so that the laser outputis a single high intensity pulse. In this embodiment a single flash lampmay be used as both the pump source and as a saturable absorber Q-switchelement.

The primary function of a Q-switch element in a pulsed laser system isto reduce the optical feedback during a selected portion of the periodduring which thelaser rod is being pumped. As the laser rod is beingpumped, the laser rod is storing energy in the form of elevated energystates of the atoms of the laser rod. When sufficient energy is storedin the laser rod, the feedback path is restored and the atomsof thelaser rod fall to a lower energy state, with the difference betweenthese energy states being radiated as light through laser action.

Each of the above discussed embodiments utilizes the change in the lighttransmitting characteristics of a gas-filled flash lamp to control theoptical feedback path of the laser system. When two flash lamps areused, one to optically pump the laser rod and the second as a Q-switchelement, as in the first embodiment discussed above, the flash lamps arepreferably connected to separate power sources with the ionization timessynchronized such that the restoration of the optical feedback loopoccurs at an optimum time after the flash lamp which is used as the pumpsource is ionized.

An object of the invention is to provide a laser system in which a flashlamp is used both as the light source and the Q-switch element.

Another object of the invention is to provide a laser system in whichthe Q-switch element is a saturable absorber.

Another object of the invention is to provide a Q- switch system havingno moving mechanical parts.

These and other objects of the invention will be more clearly understoodin view of the attached drawings and detailed descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of alaser system having separate flash lamps for the pump source and the Q-switch element.

2A 2am! 2B are waveform diagrams respectively illustrating the output ofa non-Q-switched laser and a Q- switched laser.

FIG. 3 is a schematic diagram of a laser system using a single flashlamp as both the pump source and the Q- switch element.

FIG. 4 is a schematic diagram of a laser system having two opticalfeedback loops.

FIG. 5 is a curve showing the percent of light transmitted through anionized flash lamp as a function of the intensity of the light input.

FIG. 6 is an isometric drawing of a flash lamp.

FIG. 7 is an isometric diagram of a laser system including a flash lamp,a laser rod, primary feedback mirrors, all of which are suitable for-usein the illustrated embodiments of this invention.

DETAILED DESCRIPTION The laser system which is the subject of thisinvention advantageously solves many of the problems associated withprior art Q-switched laser systems by providing a laser system in whichthe Q-switch element is a gas-filled flash lamp. One embodiment of thesystem is illustrated schematically in FIG. 1 and includes a laser rod20, a flash lamp pump source 22, a flash lamp Q-switch element 24, flashlamp trigger or coils 31 and 33, and two mirrors 26 and 28 forming theoptical feedback path. The laser rod is of neodymium dopedyttrium-aluminum-gamet (YAG), ruby, or a similar material.

The two flash lamps 22 and 24 are preferably connected to two voltagesources (now shown). One voltage source is connected to the leads 30 and32, of flash lamp 24. A second voltage source is connected to leads 34and 36 of flash lamp 22. The time at which the flash lamps ionize iscontrolled by synchronizing unit (not shown). The synchronizing unitpreferably provides two signals, one coupled to trigger coil 31 forcontrolling the ionization time of flash lamp 22, and second signalcoupled to trigger coil 33 for controlling the ionization time of flashlamp 24. As the Q-switch element, flash lamp 24, ionizes it becomesopaque and interrupts the optical feedback loop which is established bymirrors 26 and 28. The trigger signals applied to the flashlamps 24 and22 are synchronized such that the Q-switch element flash lamp 24 isionized before the flash lamp 22, which is used to optically pump thelaser rod 20. The deionization of the flash lamps is normally controlledby reducing the potential applied to the leads'of the flash lamps. Inthis system the voltage sources (not shown) are preferably adjusted suchthat the Q-switch element flash lamp 25 is deionized before the pumpsource flash lamp 22. The proper time relationship between the triggersignals and'the characteristics of the voltage sources depends on thelaser rod and the flash lamps used, and may be determinedexperimentally. FIG. 28 illustrates the output of a properlysynchronized Q-switched laser system. FIG. 2A illustrates the output ofa non-Q-switched (normal mode) pulsed laser system.

An alternate system design permits a single flash lamp to be used forthe light source and the Q-switch element. Such a system isschematically illustrated in FIG. 3 and includes a laser rod 120, flashlamp 122 mounted in a cavity structure 134, with mirrors 126, 128, 130and 132 defining an optical feedback path through the laser rod and theflash lamp 122. Mirrors 128, and 132 are fully reflecting mirrors whilethe output mirror 126 is partially reflecting.

The leads 121 and 123 of the flash lamp 122 are connected to a voltagesource (not shown). The flash lamp 122 is triggered at the system pulserate by applying a trigger signal totrigger coil 13]. When a highvoltage is applied to the leads 121 and 123 and a trigger signal to coil131 of flash lamp 122, the gas in the flash lamp 122 ionizes and thehigh voltage source discharges through it, causing it to become opaque,thereby interrupting the optical feedback path of the laser system.During this interruption, the light output of the flash lamp 122 is highand the atoms of the laser rod 120 are pumped to their higher energystate. After a population inversion of the atoms of the laser rod 120has been created by the pumping action of the ionized flash lamp, thevoltage applied to the flash lamp leads 121 and 123 is reducedsufliciently to deionize flash lamp 122. As flash lamp 122 is deionized,it becomes transparent, thereby restoring the optical feedback pathcausing the energy stored in the laser rod 120, in the form of thepopulation inversion of the atoms of the laser rod 120, to be releasedin the form of laser radiation.

Another embodiment of the invention is illustrated schematically in FIG.4 and includes a laser rod 220, mirrors 226, 228, 230, 232 and 236,flash lamp 222, and a laser cavity structure 234. The output mirror 226and the auxiliary feedback mirror 236 are partially reflecting. The backmirror 232 and the reflecting mirrors 228 and 230 are fully reflecting.This arrangement provides two optical feedback paths; the first orprimary feedback path extending from the output mirror 226 to the backmirror 232, the second or auxiliary feedback path extending from theoutput mirror 226 to the auxiliary mirror 236.

The flash lamp 222 serves as a light source to pump the laser rod andalso as a saturable absorber Q-switch element. As previously discussed,the essential characteristic of a saturable absorber Q-switch element isthat the Q-switch element mustbecome increasingly transparent as theintensity of the light radiation impinging upon the Q-switch elementincreases.

In the system illustrated in FIG. 4, the flash lamp is ionized byplacing a proper voltage between the leads 221 and 223 of flash lamp 222and applying a trigger voltage to the trigger coil 231. As the flashlamp ionizes, the light output of the flash lamp increases with aportion of this light being absorbed by the laser rod 220, therebycausing the atoms of the laser rod to change from a low level energystate to a high level energy state. Some of the atoms of the laser rodwill spontaneously change from their high energy state to their lowenergy state by the spontaneous radiation process and emit a smallamount of light. Some portion of this light will emerge from the laserrod 220 along the longitudinal axis of the laser rod and impinge on theoutput mirror 226 or the auxiliary mirror 236. In either case, a smallportion of this light will be reflected back through the laser rod alongits longitudinal axis and cause additional light to be emitted from thelaser rod along the longitudinal axis by the process of stimulatedemission of radiation. This process continues and the intensity of theradiation in the feedback loop increases with each pass of the lightthrough the laser rod 220. A portion of the light impinging upon outputmirror 226 is transmitted through this mirror as an output of the systemwhile a portion of the light impinging on auxiliary mirror 236 passesthrough this mirror and is By adjusting the reflectivity of the outputmirror 226 and the auxiliary mirror 236 and the distance between thesemirrors, the overall system feedback can be made very low during theearly portion of the pumping cycle and very high during the latterportions of the cycle due to the saturable absorption characteristics ofthe ionized flash tube. As previously discussed this change in feedbacklevel is the principal function of a Q-switch element. It is noted thatfor some applications, it may be advantageous to interchange mirrors 232and 226.

The above discussed system depends on the saturable absorptioncharacteristics of an ionized flash lamp. A generalized characteristicfor such a lamp is shown in FIG. 5. The curve shown in FIG. 5 wasmeasured for a typical xenon filled flash lamp. The characteristics of aparticular lamp will vary with the structural details of the particularlamp; however, the saturable absorption characteristic may be aninherent feature of all gas filled flash lamps.

FIG. 6 illustrates a flash lamp suitable for use in the above discussedlaser systems. The flash lamp is conventional in design; however, theendplates of the flash lamps 322 and 324 preferably have reasonably goodoptical qualities because the optical feedback path of the laser systemis through these plates. For example, end plates with low transmissionratios or uneven surfaces tend to decrease the optical feedback of thelaser system.

FIG. 7 illustrates a laser cavity structure 334 in which a laser rod 320and a flashlamp 322 having trigger coil 331 are mounted. The innersurfaces of the cavity are essentially elliptical in shape with theflash lamp 322 and the laser rod 320 mounted at the face of the foci.This mounting of the laser rod 320 and the flash lamp 322 assures thatthe light emitted by the flash lamp 322 will be focused on the laser rod320. The inner surface of the cavity is preferably highly reflective.For example, this high reflectivity may be achieved by a highly polishedcoating of silver on this surface. Other coatings may also be used. Alsoincluded is an output mirror 326, two reflecting mirrors, 328 and 330,and a back mirror 332. The cavity structure 334, the flash lamp 322, thelaser rod 320, and the mirror 326, 328, 330 and 332 are suitable for usein any of the disclosed embodiments of this invention. Alternately, themirrors might be incorporated by being formed directly on an end of thelaser rod or an end of the flash lamp.

No general theory has been developed to adequately explain the lightblocking characteristic of an ionized flash lamp for relatively lowlevels of light or thelight saturable characteristic for higher levelsof light. However, it has been shown that these characteristics can beused as an effective Q-switch in pulsed laser systems.

The present invention has been described and defined in detail andillustrated in preferred embodiments. It will be apparent, therefore, toone skilled in the arts herein encompassed that many changes andmodification are possible within the ordinary skill of such artisanswithout departing from the scope of the invention as described anddefined.

What is claimed is:

1. A Q-switched pulse pumped laser system comprismg:

a. an optical cavity;

b. a laser flash tube mounted in the optical cavity for opticallypumping and Q-switching the laser;

c. a laser rod mounted in the optical cavity responsive to the output ofthe laser flash tube for generating stimulated emission energy pulses;and

d. an optical feedback means for feeding back the stimulated emissionenergy through the laser rod and laser flash tube.

2. A laser system according to claim 1 wherein said optical feedbackmeans includes reflector means positioned in the feedback pathbetweenthe laser rod and the laser flash tube for directing thestimulated emission energy received from the laser rod through the laserflash tube for Q-switching.

3. A laser system according to claim 2 wherein said optical feedbackmeans further includes a partially reflecting means in the feedback pathbetween the laser rod and the laser flash tube for selective feedback ofspontaneous emission energy to the laser rod for con trolling the timingof Q-switching.

4. A laser system according to claim 1 wherein said cavity includes ahighly reflective inner surface.

5. A laser system according to claim 4 wherein said highly reflectiveinner surface comprises a high polished coating of silver.

6. A laser system according to claim 5 wherein said cavity inner surfaceis elliptically shaped and said laser flash tube and laser rod aremounted at the face of the foci.

1. A Q-switched pulse pumped laser system comprising: a. an opticalcavity; b. a laser flash tube mounted in the optical cavity foroptically pumping and Q-switching the laser; c. a laser rod mounted inthe optical cavity responsive to the output of the laser flash tube forgenerating stimulated emission energy pulses; and d. an optical feedbackmeans for feeding back the stimulated emission energy through the laserrod and laser flash tube.
 2. A laser system according to claim 1 whereinsaid optical feedback means includes reflector means positioned in thefeedback path between the laser rod and the laser flash tube fordirecting the stimulated emission energy received from the laser rodthrough the laser flash tube for Q-switching.
 3. A laser systemaccording to claim 2 wherein said optical feedback means furtherincludes a partially reflecting means in the feedback path between thelaser rod and the laser flash tube for selective feedback of spontaneousemission energy to the laser rod for controlling the timing ofQ-switching.
 4. A laser system according to claim 1 wherein said cavityincludes a highly reflective inner surface.
 5. A laser system accordingto claim 4 wherein said highly reflective inner surface comprises a highpolished coating of silver.
 6. A laser system according to claim 5wherein said cavity inner surface is elliptically shaped and said laserflash tube and laser rod are mounted at the face of the foci.